1. Field of the Invention
The invention is directed to a method for determining the gain factor of a photomultiplier with a radiation source arrangement that emits gamma quanta, a scintillation arrangement that absorbs incident gamma quanta and thereby emits photons, with the photomultiplier being optically coupled to the scintillation arrangement and which emits an output signal dependent on incident photons.
2. Description of the Prior Art
The use of gamma cameras in nuclear medicine to detect radioactive indicators in vivo and in vitro is an important area of employment of photomultipliers. The gamma quanta emitted by the radioactive indicators are thereby absorbed in a scintillation arrangement. Excited by the absorption of the high-energy radiation, a number of photons, proportional to the average energy, is emitted at the absorption or scintillation location, these photons being acquired by an arrangement of photomultipliers. Dependent on the incident photons, the photomultipliers output an electrical output signal that is employed for localizing (identifying the location of the source of) the scintillation event. Photomultipliers are employed because they can deliver an output signal which is only slightly contaminated with noise, given a high gain of more than 10.sup.6.
An extremely critical factor in the employment of photomultipliers in a gamma camera is that the respective gains--i.e. the relationship between the triggered electrons and the signal--are the same for all of the photomultipliers or are at least known for each individual photomultiplier, for the following reasons. The image quality of the gamma camera is based, first, on the suppression of gamma quanta having energy outside a predetermined value or range and, second, on the correct reconstruction of the absorption location in the camera. For this first task, an aggregate signal of all signals emitted by the photomultipliers must be formed and accepted or discarded with respect to its pulse amplitude. The signal-to-noise ratio in the image is generally established by this operation. For the source location reconstruction, the signals of the respective photomultipliers must be weighted and added, or otherwise processed in a digital camera with special algorithms. The calibration of the photomultipliers thereby defines the linearity of the imaging, and thus the precision of the measured activity distribution. The calibration of the photomultipliers is critical for determining the energy as well as for the correct reconstruction of the absorption location of a scintillation event. All photomultipliers, as well as the following, analog amplifiers, should have the same gain insofar as possible, or correction factors for each photomultiplier should be known. The actual gain of the photomultipliers must be measured for that purpose.
In a known calibration method for the gain, the camera head of the gamma camera is irradiated with a point source arranged at a distance of about 1.5 m. The photomultipliers of the camera are individually selected via a multiplexer. The signals of the selected photomultiplier are then digitized. The absorption locations of the gamma quanta are thereby randomly distributed over the entire area or surface of the camera head and--with reference to the photomultiplier selected at the moment--all possible spacings of the absorption location and thus a different number of photons per scintillation event, occur. In order nonetheless to obtain a spectrum with an unambiguous photopeak for each photomultiplier, a localizing procedure is utilized in order to select only those scintillation events that fall into a so-called "tune mask area" that generally lies under the selected photomultiplier. The high-voltage of the photomultipliers is then adjusted such that the photopeak lies symmetrically relative to a pre-set energy window. A disadvantage of this technique is that the localizing procedure may be implemented with a possibly uncalibrated photomultiplier, so that the "tune mask area" is also shifted relative to the symmetry axis of the photomultiplier. The photopeak of the events from the "tune mask area" is thus also systematically shifted. In principle, this method can only function iteratively with the calibration ensuing in a number of steps.
In a calibration method disclosed IN U.S. Pat. No. 4,228,515, a camera head having a group of photomultipliers is irradiated with a point radiation source via a collimator. The location of the source is thereby known. The signals of the photomultipliers are digitized and then normalized for each position of the point source. The normalized measured values are then averaged. Additionally, the standard deviation of the normalized measured value compared to the average, normalized measured value is determined for the individual photomultipliers for each position. The slope of the signal responses is calculated for each photomultiplier from the average, normalized signals given a plurality of positions of the radiation source. The relationship between slope and standard deviation yields a weighting factor for each position for the uncertainty or certainty of the signal responses of each photomultiplier. These weighting factors are utilized in the localizing of the scintillation events. The gain of the photomultipliers themselves, however, is not identified with this known calibration method.